In high speed applications such as analog to digital conversion in image processing systems, it is often necessary to produce a current from a voltage such that the current is precisely proportional to the voltage despite drift due to time and temperature and distortion due to switching currents interfering with the switched current. These problems are especially serious when the switching times are in the picosecond range to provide integration conversions in the nanosecond range.
A further problem occurs when the integration window must be capable of being varied over a wide range, for example from 6 to 48 nanoseconds: at shorter durations, less charge is accumulated and so a lower voltage is applied to the A/D converter so that the whole input range of the A/D converter is not fully utilized.